The present invention relates to a process for regenerating a catalyst used in hydrocarbon conversion, more particularly, it relates to a process for continuously regenerating catalyst particles.
Catalytic reforming is an important technology or producing high octane gasoline and aromatics, and by-product hydrogen, wherein the reactions taking place include dehydrogenation, dehydrocyclization, isomerization, cracking and coking, etc, The coke formed by coking deposits on the surface of the catalyst and causes it to deactivate, so there is a need to regenerate the catalyst and restore its activity through regeneration. Regeneration generally includes coke-burning, oxychlorination, calcination and reduction. Coke-burning is to burn off the coke deposited on the catalyst and carry away the heat generated in burning with an oxygen-containing gas. Oxychlorination is to complement the chlorine component lost from the catalyst and oxidize the active metal components and uniformly distribute them on the surface of the catalyst support. Calcination is to remove water contained in the catalyst. Reduction is to reduce active metal components in oxidized states in a hydrogen atmosphere.
Currently, a radial bed is adopted in the main-body configuration of the coke-burning zone of a continuous reforming regenerator in industry, wherein the catalyst particles slowly move downwards in the annular bed by means of gravity, and an oxygen-containing regeneration gas passes the catalyst bed along the radial direction, thereby realizing the continuous coke-burning.
The service life of the reforming catalyst mainly depends on the decreasing rate of its specific surface area and the major factors that affect the decreasing rate of the specific surface area arc the moisture of the regeneration gas, the regeneration temperature, and the residence time of the catalyst within the high temperature zone. It has been pointed out by O. Clause et al in “Continuing Innovation in Cat Reforming (1998 NPRA, AM-98-39) that the loss of the specific surface area of the catalyst reduces along with the decrease of water content in the regeneration gas. The trace amount of oil vapor adsorbed by the carbon-containing deactivated catalyst from the reaction system and the coke deposited on the catalyst will generate a great amount of steam during burning reaction, thereby resulting in a relatively high content of water in the regeneration gas, Besides, an environment of high temperature and high moisture will exert an adverse effect on the physical properties of the catalyst because coke-burning is carried out at a high temperature, thus causing a loss of the specific surfaces area of the catalyst and the agglomeration of platinum particles and affecting the activity of the catalyst. In the meanwhile, the presence of a large amount of steam will speed up the loss of the acidic component, chlorine, from the catalyst.
For a radial coke-burning bed, at the inlet of the bed when the catalyst having a high carbon content contacts the oxygen-containing regeneration gas transversely passing the catalyst bed, a great amount of heat is released from coke-burning. Such heat congregates gradually towards the inner screen and causes the temperature at the upper part of the bed to rise, while the temperature at the lower part of the bed rises slightly because the content of carbon in the catalyst is low and less heat is released. Therefore, the radial bed has the disadvantage of the irrationality of the temperature distribution throughout the whole bed. The high temperature zone in the upper part of the bed will exert an adverse effect on the catalyst performance, while the temperature at the lower part is relatively low and there is a potential to further enhance the capacity of coke-burning. Consequently, proper adjustment of the temperature distribution within the bed will have a positive effect on protecting the catalyst performance and prolonging its service life.
The early form of the coke-burning zone of a continuous reforming regenerator is a strip-shape one. For example, in the technologies provided in U.S. Pat. No. 3,692,496, U.S. Pat. No. 3,725,249, U.S. Pat. No. 3,761,390, and U.S. Pat. No. 3,838,038, the catalyst particles slowly move downwards within a strip-form space by means of gravity, and the regeneration gas is introduced from one side and withdrawn from the other side. After passing through an caustic scrubbing unit and a regeneration gas blower, the regeneration gas returns to the coke-burning zone of the regenerator for recycle use. Oxygen required for coke-burning is supplemented partially by the oxygen-containing gas from the oxychlorination zone. Since there is no drying system in the recycle loop of the regeneration gas of the above patents, the water content in the regeneration gas is relatively high, thereby affecting the service life of the catalyst.
In the thermal regeneration technology provided in U.S. Pat. No. 4,578,370, the coke-burning zone in the regenerator is a section of radial bed configuration and the catalyst particles slowly move downwards in an annular space by means of gravity. The gas space between the outer screen of the coke-burning zone and the inner wall of the regenerator is divided into two parts. After collected in the central pipe, the regeneration gas is withdrawn from the regenerator, a small portion of which is vented and the remainder passes through a regeneration gas blower and is divided into two parts. One part enters the upper coke-burning section via an air cooler and a heater, and the other part directly enters the lower coke-burning section. Since no drying system is provided in the recycle loop of the regeneration gas, the water content in the regeneration gas is adjusted by supplementing air and venting regeneration gas to finally reach a balanced value. The content of water in the regeneration gas is always maintained at a relatively high level, thus exerts an adverse effect on the catalyst performance.
In the regeneration technology provided in U.S. Pat. No. 4,859,643 and U.S. Pat. No. 5,277,890, the coke-burning zone of the regenerator has a tapered configuration. The bed has different thickness at different axial positions. This can improve the gas distribution along the axial position. The upper part of the bed is thinner and the amount of distributed gas is greater, while the lower part of the bed is thicker and the amount of distributed gas is less, thus better satisfying the requirement for oxygen at different axial positions and reducing the residence time of the catalyst in the high temperature zone in the upper part of the bed. However, since no drying system is provided in the recycle loop of the regeneration gas, the content of water in the regeneration gas is relatively high.
In the thermal regenerating technology provided in U.S. Pat. No. 4,880,604 and U.S. Pat. No. 4,977,119, the coke-burning zone of the regenerator has a strip-shape configuration. The catalyst particles slowly move downwards within the strip-shaped space by means of gravity. The upper part and the lower part of the outer screen have different perforation rates, thereby permitting different distributions of the regeneration gas along the axial direction, so that a greater amount of the gas is distributed to the upper part, while less amount of the gas is distributed to the lower part, so it is beneficial to meeting the needs for oxygen in different axial positions. However, since no drying system is provided in the recycle loop of the regeneration gas, the content of water in the regeneration gas is relatively high.
In most of the technologies introduced in the above patents, the contents of water in the recycled regeneration gases are all relatively high and the coke-burning for regenerating the catalyst particles is carried out in an environment of high temperature and high water content. Such an environment is easy to cause a loss of the specific surface area of the catalyst, thereby shortening its service life. Moreover, there commonly exists the problem of high temperatures at the inner screen of the upper part of the radial bed, Although U.S. Pat. No. 4,859,643, U.S. Pat. No. 5,277,880, U.S. Pat. No. 4,880,604, and U.S. Pat. No. 4,977,119 propose a bed configuration having a tapered configuration and different perforation rates and thereby increase the amount of oxygen required in the upper part of the bed, but consequently the temperature near the inner screen in the upper part of the bed is raised and therefore the problem of the irrationality of the temperature distribution in the radial bed has not yet been solved.
In the dry regeneration technology provided in U.S. Pat. No. 5,034,177, the catalyst bed in the coke-burning zone is divided into two sections which are same in the configuration and size but somewhat different in the conditions at the inlets, namely the inlet temperature of the regeneration gas in the second section of the bed is higher than that in the first section of the bed and air is supplemented via the space between the two sections to maintain the oxygen content at a level as required in each section respectively. After passing through the first and second coke-burning sections in sequence, the regeneration gas is withdrawn from the regenerator and mixed with the outlet gas from the oxychlorination zone. Said regeneration gas is then introduced into the scrubbing and drying system and sent back to the first coke-burning section of the regenerator through the recycling compressor. Although the water content in the recycled regeneration gas entering the regenerator is relatively low because of the drying system provided in the recycle loop of the regeneration gas, the catalyst is still in an environment of high temperature and high water content because when the regeneration gas enters the second coke-burning section directly from the first coke-burning section, the steam generated in the first coke-burning section in the upper part by the burning reaction of the small amount of hydrocarbons and hydrogen in coke carried by the deactivated catalyst also enters the second coke-burning section. Such an environment may result in a rapid decrease in the specific surface area of the catalyst and affect its service life.
In summary, there are mainly two types of coke-burning zones in the regenerators of the prior arts. One is a two-sectioned radial bed with a drying system provided in the loop of the recycled regeneration gas, and the other is a one-sectioned radial bed without a drying system in the loop of the recycled regeneration gas. In the former, the steam generated by the coke-burning in the upper part of the bed entirely enters the lower coke-burning section, resulting in that the final coke-burning is completed in an environment with high water content. In the latter, the water content in the regeneration gas is even higher and the temperature near the inner screen in the upper part of the bed is relatively high, so the coke-burning is completed in an environment of high temperature and high water content. As a result, both of the above types of coke-burning have the problem that the catalyst is in an environment of high temperature and high water content, which affects the service life of the catalyst.
The object of the present invention is to provide a process for continuously regenerating catalyst particles under an environment of lower temperature and lower water content compared with the prior arts.